Do I need a dehumidifier?

[Manny]

Or am I wasting my time?

We renovated the cellar in our terrace in the autumn of last year. It looked dry enough and I did some testing with a rubber mat to ensure that water wasn't coming in then evaporating. We wallpapered, layed a new timber floor and moved some furniture down there.

Following the humid weather we're having now and coupled with the fact there isn't many air changes/hour down there, we're getting some white furry patches on the walls after a time. These rub off easy enough, the real problem is that in some places wallpaper is starting to bubble a little and the furniture is damp. The wooden floor has also started to blow and buckle though the effects of the higher moisture content of the air.

I'm guessing that humidity control would be the way forward, however the room has an open fire. Would I just be trying to dry out the whole sky?

 

[Penners]

Following the humid weather we're having now and coupled with the fact there isn't many air changes/hour down there, we're getting some white furry patches on the walls after a time.

It's quite possible that you've actually answered your own question. The walls and floor of your cellar are probably cold, because of their proximity to damp subsoil, and the recent warm humid weather has caused extensive condensation on them.

Dehumidifiers can be a double-edged sword. Yes, they'll dry out the air, but if they cause excessive drying out of the walls and floor, this could draw in moisture through these structures.

If I were you, I would see about arranging for the cellar to be better ventilated, so that air can move through it effectively. You can buy ventilation fans that are automatically controlled by a humidistat. This might be the best way to go.

 

[Andrew Bodenham]

Good ventilation will certainly help, but you need to think carefully about where you are moving air to and from. For example, if you arrange a simple extractor fan in the cellar, then where will the air come from to replace the air which the fan is blowing outside? Perhaps it will get drawn down from the house above. This will be warm air (good) but probably carrying a lot of water vapour with it, which might well condense on the cold surfaces in the cellar (bad).
Maybe you could arrange for an inlet vent from outside on one side of the cellar, and extract via a fan on the other side, so drawing cold dry air through the room. Adding heat in the cellar would help, but then you'd be pumping your expensive warm air outside. More to this than meets the eye, I feel.

You mention that the cellar has a fireplace - presumably with an open flue. This should provide you with good ventilation - essentially it's what is known as Passive Stack Ventilation. If you can make the cellar air temp warmer than outside then you'll create an airflow in the flue. You still need to think about where the replacement air is coming from.

Alternatively you could seal up the room and run a dehumidifier to dry out the air - they also have a warming effect which would be beneficial. This is a rather energy intensive approach, but will reduce the moisture content of the air and hence the fabric and decor. The problem is that drying out the cellar will probably increase the rate of water ingress from the surrounding ground.

Unfortunately cellars are inherently cold, damp places. The only way to achieve 'house-like' levels of warmth and dryness would be to apply extensive modern barrier techniques such as tanking.

 

[MdB]

some white furry patches on the walls after a time.

This could well be salt rather than mold which would suggest water moving through the walls an evaporating.

 

[Lime]

Cellars vary but are usually more subject to the weather than anywhere else in the house.
Lots of rain or very dry weather can completely alter your cellar maybe for many years to come.
Being below ground level (usually) they are prey to the water table and any water running underground and this can change for many reasons, most out of your control.
My experience with cellars has been in clay.
What the earth consists of makes a great difference as to what you can expect from your cellar.
IME large amounts of rain can cause water several inches deep to suddenly appear in a previously dry cellar, so count your blessings.
All the advice given so far makes sense.
The air needs to be changed and replaced with air that is cooler than in the house.
Cooler outside air will hold less moisture usually and much less than air from inside the house.
Even if the outside air is saturated with water, on entering the cellar it will warm slightly and be able to hold more moisture.
Warm air from the upper part of the house will cool and be less able to take up any moisture and is quite likely to deposit moisture as condensation as the air reaches saturation.
Remember the air needs an entrance and an exit.

 

[Haresy77]

I used a dehumidifier down a cellar for about 6 months in my last house. It worked well in keeping any storage items dry but it didn't half draw the moisture through the walls and I was taking about a litre a day out - and this was a cellar that was relativley dry anyway. I got heavy salt deposits and even a coat of lime wash I'd put on crumbled and fell off in large patches. In hindsight, the dehumidifier I was using may have been too powerful so be careful. One which turns off once the hunidity reaches an acceptable level would be best. I remember an old post where quite a debate took place on this subject and got quite technical but this may have been before the new forum.

 

[sarahj]

A litre a day in the cellar.... cripes, what does that say about my damp underground dining wall - we're getting FIVE litres a day!!

And my partner is back on his inhaler after removing the plasterboard and finding all the black mould!

sarah